A reproduction device, which reproduces digital video and audio data stored in media such as a CD, DVD, or hard disk (HD), is usually coupled to output devices, such as a monitor and/or a speaker, so that digital video and audio contents can be enjoyed. For example, there is a system that couples a plurality of input devices (i.e., reproduction devices) to a single output device and switches the output data. A system that couples a plurality of input devices and a plurality of output devices and switches the input data and output data has also been proposed to increase system versatility. In such a system, data transmission is in compliance with the IEEE 1394 standard, which is an international standard used for serial data communication systems.
IEEE 1394 enables high-speed data communication at 400 Mbps. For continuous moving image and audio reproduction, IEEE 1394 specifies an isochronous transfer mode, in which data transmission is performed in a given cycle (for example, 125 μs, hereinafter referred to as isochronous (ISO) cycle), and an asynchronous transfer mode, in which data transmission is performed asynchronously. The isochronous transfer mode is suitable for the transfer and reproduction of data such as digital video and audio data.
For example, in the system illustrated in FIG. 1, a transmission device 100 is coupled to a CD reproduction device 101, a microphone 102, and a DVD reproduction device 103, each of which serves as an input device. The transmission device 100 is coupled to different reception devices (not shown), each corresponding to the CD reproduction device 101, the microphone 102, and the DVD reproduction device 103, by a bus cable.
The transmission device 100 stores information (transfer destination information) for each input device to output the data input from each of the input devices 101 to 103. A path for transferring the data from the input device to an output device (coupled by a reception device) is referred to as a channel. That is, three channels are formed in the system shown in FIG. 1.
The transmission device 100 stores the data input from each of the devices 101 to 103 to an input unit 104 in a first-in first-out (FIFO) memory 105. The transmission device 100 generates packets including given amounts of data with a packet generator 107 in accordance with the data size stored in a register 106.
Referring to FIG. 2, a transmission channel controller 108 receives a cycle start packet for the isochronous transfer mode (operation 121) and sequentially enables the three channels (operation 122). In accordance with the quantity of the channels that is enabled, a channel selector 109 outputs the packet generated by the corresponding packet generator 107 through an output unit 110. The transmission channel controller 108 waits for the next cycle start packet after all the channels have been enabled.
FIG. 3 chronologically illustrates the packets output from the transmission device 100 to the bus cable in time-series. In FIG. 3, sections divided by broken lines 131 represent isochronous (ISO) cycles. Packets P1, P2, and P3 for every one of the channels are transferred in each ISO cycle. The packet P1, which is transmitted from the CD reproduction device 101 to the output device, includes a header D1a, which is packet information, packet data D1b, which is a data portion of the packet, and a footer D1c, which is packet information. The packet P2, which is transmitted from the microphone 102 to the output device, includes a header D2a, packet data D2b, and a footer D2c. The packet P3, which is transmitted from the DVD reproduction device 103 to the output device, is formed by a header D3a, packet data D3b, and a footer D3c. 
Japanese Laid-Open Patent Publication No. 2001-94575 describes a system that uses the isochronous transfer mode.
With regard to digital video data, the image quality has been improving and channels have increased. This has increased the amount of transmitted data. However, the bands for data transmittable by a transmission device have become insufficient. Thus, there is a shortcoming in that data cannot be transmitted.